A vehicle seat is provided with a seat belt device for restraining the vehicle occupant to prevent the vehicle occupant from being thrown forward or sideways in case of a vehicle crash. Since such a seat belt device is desired not to obstruct the normal movement of the occupant and to be tightened only when a deceleration in excess of a prescribed value has been applied to the vehicle, there have been proposed a number of seat belt tightening devices which detect an impact of a vehicle crash through a displacement of an elastically supported pendulum caused by an inertia force acting thereon, ignite a propellant which can rapidly expand by a chemical reaction by the action of a trigger engaged by the pendulum, and wind the seat belt by way of a cable which is connected to a piston received in a cylinder so that the thrust due to the rapid expansion of the propellant may be applied to the piston. For more detailed discussion of this technology, reference is made to copending U.S. patent applications Ser. No. 07/030,630 (filed Mar. 27, 1987), now U.S. Pat. No. 4,840,325, dated June 20, 1989; Ser. No. 07/132,257 (filed Dec. 14, 1987), now U.S. Pat. No. 4,864,086, dated Sept. 5, 1989; and Ser. No. 07/250,782 (filed Sept. 28, 1988; now U.S. Pat. No. 4,955,638, dated Sept. 11, 1990, assigned to the assignee of the present application.
Since such a device is used in a highly unusual situation such as in case of a vehicle crash, and it is virtually impossible to test its capability to operate under normal condition, an extremely high operation reliability is required. Thus, the strength of the coupling between the piston which receives the expansion pressure of the propellant and the cable connected therewith is highly important. Normally, the cable and the piston are connected to each other by passing an end of the cable into a hole extending through the piston and crimping the end of the piston opposite to the pressure receiving end. Therefore, the length of the piston must be increased to accommodate this crimped part, and it is difficult to ensure a sufficient and stable strength of coupling between the cable and the piston.
Further, the fit between the cylinder and the piston is important in efficiently applying the expansion pressure of the propellant to the piston and ensuring a smooth sliding movement of the piston in the cylinder at the same time. To achieve a precise fit, manufacturing precision must be increased but it leads to the increase in manufacturing cost and causes an added complexity to quality control.